1. Field of the Invention
The present invention relates to a sampling rate converting system for applying a sampling rate conversion to an input image signal to display the input image signal on a display on which pixels each formed of a plurality of subpixels are arranged in a matrix fashion, and a filtering method applied to the sampling rate converting system.
2. Description of the Related Art
Out of various display devices (displays), in the display in which the pixels are arranged in a matrix fashion and are caused to emit the light in predetermined sequence, each pixel corresponds to each signal data value. For example, the flat panel display (FPD) such as PDP (Plasma Display Panel), LCD (Liquid Crystal Display), and the like applies to such display.
In these displays, as shown in FIG. 1A, three light emitting elements (subpixels) corresponding to three primary colors of R(red), G(green), B(blue) are aligned, and the luminance and the color are reproduced while using these elements as one pixel. Then, as shown in FIG. 2, the image display device having such display includes a number-of-pixels converting portion 101, a matrix portion (YUV→RGB) 102, and a panel 106. The number-of-pixels converting portion 101 adapts the number of pixels of an input signal to the number of pixels of the display based on the number-of-pixels conversion. In some cases the number of pixels of the signal agrees with the number of pixels of the display. However, since the signal source is diverse nowadays, such number-of-pixels converting portion 101 is substantially indispensable to the display.
Here the input signals consists of the luminance signal Y and the color difference signals U, V as the transmission signals for the television. In the color difference signals (U, V signals) as the standard signal (SD signal) based on NSTC (National Television Standard Committee), or the like, the color difference signal (B-Y) is called Cb and also the color difference signal (R-Y) is called Cr. Also, in the high definition signal (HD signal), the color difference signal (B-Y) is called Pb and also the color difference signal (R-Y) is called Pr. Since the transmission matrix coefficient values are different between the standard signal and the high definition signal in connection with the colorimetry parameter, the color difference signals are distinguished one from the other. Now, as the general name of the color difference signals, the color difference signal (B-Y) is called a color difference signal U and also the color difference signal (R-Y) is called a color difference signal V.
The input signals Y, U, V are subjected to the number-of-pixels conversion by the number-of-pixels converting portion 101, then are converted into the RGB signals by the matrix portion 102, and then are transferred to the panel 106. The panel 106 has a memory function, and respective pixels on the panel 106 emit light according to respective signal data values. Here, the memory function of the panel 106 is accomplished by a physical memory function of the panel 106 and an electronic frame or line memory, and signifies that RGB pixel data 103, 104, 105 reproduce a tone to correspond to respective pixels on a one-to-one basis.
FIG. 3A is a view showing the model applied to the number-of-pixels conversion in a prior art. Here high-definition luminance signals in one frame, which consists of 1920 pixels in the horizontal direction and 1080 pixels in the vertical direction, are converted to correspond to a display, which consists of 853 pixels in the horizontal direction and 480 pixels in the vertical direction.
As the frequency characteristic after the number-of-pixels conversion when the horizontal direction is observed, as shown in FIG. 4A, the sampling frequency (sampling rate) is given as 853 cycle/line (abbreviated to “cpL” hereinafter) and also the Nyquist limit, which is defined as an upper limit of a reproducible range of the signal, is given as almost 427 cpL, which is ½ of the sampling frequency, based on the sampling theorem. In contrast, the sampling frequency of the high-definition signals before the number-of-pixels conversion is given as 1920 cpL and also the Nyquist limit is given as 960 cpL.
FIG. 5 is a view explaining the concept of the number-of-pixels conversion in the prior art. In FIG. 5, the sampling frequency of the high-definition luminance signal is converted from 1920 cpL to 853 cpL. At this time, the filtering process for suppressing an aliasing to attain phase matching is carried out. Similarly the sampling frequency conversion and the filtering process are applied to the color difference signals. Here, in compliance with normal transmission signals, the sampling frequency thereof is set to a sampling frequency (960 cpL) that is ½ of that of the luminance signal. In some cases the color difference signals have the same number-of-pixels (sampling frequency) as the luminance signal.
Then, in case the filtering characteristics in the number-of-pixels conversion are extended to the high frequency range, the aliasing occurs about the Nyquist limit and acts as the interference, as shown in FIG. 4A. If the bandwidth in the number-of-pixels conversion is suppressed to avoid generation of the aliasing, the resolution is deteriorated and the picture quality is worsened.
In Japanese Patent Application Laid-open No. 2003-187243, a display method utilizing three subpixels of RGB to improve display quality is set forth. According to this method, different brightness components are applied to three subpixels constituting one pixel. As a result, brightness information can be reflected in the produced image in units of subpixel, and thus the display quality can be improved.
Also, in Japanese Patent Application Laid-open(KOHYO) No. 2003-520507, a method utilizing subpixels in the conversion from the standard definition (SD) signal to the high definition television (HDTV) signal is set forth. According to this method, when an edge of an image is sensed during the signal conversion from the SD signal to the HDTV signal, the transient characteristic of the luminance signal can be improved at a subpixel level.
However, in the methods set forth in Japanese Patent Application Laid-open No. 2003-187243 and Japanese Patent Application Laid-open(KOHYO) No. 2003-520507, the number-of-pixels conversion applied when the number of pixels of the input image signal (signal source) is larger than the number of pixels of the display is not taken into account at all. As a consequence, even though the methods set forth in Japanese Patent Application Laid-open No. 2003-187243 and Japanese Patent Application Laid-open(KOHYO) No. 2003-520507 are applied, as they are, to the number-of-pixels conversion applied when the number of pixels of the input image signal is larger than the number of pixels of the display, the problem such as generation of the above aliasing interference or deterioration of the image cannot be overcome.